1. Field of the Invention
This invention relates to a misfire detection system for a vehicle multicylinder internal combustion engine, particularly to a system for detecting misfire occurred in an internal combustion engine having a plurality of cylinders.
2. Description of the Related Art
Misfiring of an internal combustion engine installed in a vehicle degrades the engine""s performance and fuel efficiency and also adversely affects the exhaust gas purifier owing to after-firing of the unburned gas in the emission control system. Early detection of misfire is therefore desirable.
However, vehicles are driven under various conditions, and if misfire detection is conducted during vehicle running on a rough road with bumps and recesses, for example, false detection is apt to occur. This makes it necessary to accurately discriminate the rough road and other specific running (operating) conditions likely to cause erroneous misfire detection and to disable misfire detection when such a condition arises. Japanese Patent No. 2,976,684, for example, teaches a technology for this purpose which measures the period of a pulse signal output by a wheel speed sensor, calculates the average value for every prescribed number of pulse signals, calculates the difference between the preceding and current average values to determine the amount of fluctuation, compares the amount of fluctuation with a prescribed value to determine whether the vehicle is experiencing a rough road running condition or other such specific running (operating) condition that affects misfire detection, i.e., is likely to cause false misfire detection, and disables the misfire detection when such a running (operating) condition is found to be present.
However, this prior-art technology is limited to discrimination based on the amount of fluctuation in the measured value of the wheel speed sensor""s output pulse signal period. It is therefore incapable of thoroughly distinguishing between wheel speed fluctuation caused by misfire and wheel speed fluctuation caused by a specific running (operating) conditions such as rough road running. The accuracy of the discrimination as to whether the vehicle is experiencing rough road running or other such specific running (operating) conditions is therefore often unsatisfactory.
An object of this invention is therefore to overcome the aforesaid problem by providing a misfire detection system for a vehicle multicylinder internal combustion engine that can reliably avoid false misfire detection by accurately discriminating rough road running and other specific running (operating) conditions that adversely affect misfire detection.
In a first aspect, the present invention achieves the foregoing object by providing a system for detecting misfire occurred in an internal combustion engine having a plurality of cylinders and installed in a vehicle, comprising: an engine speed sensor that outputs a signal at every prescribed crank angle indicative of a speed of the engine; misfire detection means for detecting misfire occurred in individual cylinders of the engine based on the rotation signal outputted by the engine speed sensor; a wheel speed sensor that outputs a signal indicating a rotation speed of a wheel of the vehicle; first fluctuation value calculation means for calculating a first fluctuation value of the wheel rotation speed based on the signal outputted by the wheel speed sensor through a first filter; second fluctuation value calculation means for calculating a second fluctuation value of the wheel rotation speed based on the signal outputted by the wheel speed sensor through a second filter; specific running condition discrimination means for calculating a parameter indicative of a degree of fluctuation of the wheel rotation speed based on the calculated first fluctuation value and second fluctuation value, and for discriminating whether the vehicle is under a specific running condition that affects the misfire detection of the misfire detection means based on the calculated parameter; and misfire detection disable means for disabling the misfire detection of the misfire detection means when the specific running condition discrimination means discriminates that the vehicle is under the specific running condition.
Thus, a rough road running condition or other such specific running (operating) condition affecting misfire detection can therefore be accurately detected to reliably avoid false detection of misfire while ensuring that misfire detection is not erroneously disabled when misfire does occur.
In a second aspect of the present invention, the first filter is a band-pass filter that passes a first prescribed frequency component in the output signal of the wheel speed sensor, and the second filter is a band-pass filter that passes a second frequency component that is higher than the first prescribed frequency and that is not a multiple of the first prescribed frequency.
By this, the eigenfrequency of the vehicle drive train can, for example, be selected as the prescribed frequency component to be extracted by the first filter, and a frequency component at which the effect of misfire does not easily appear in the output signal of the wheel speed sensor can, for example, be selected as the frequency that is higher than the prescribed frequency and not a multiple thereof to be extracted by the second filter. Solely the fluctuation of the wheel speed sensor output signal attributable to the rough road can therefore be extracted, i.e., fluctuation of the wheel speed sensor""s output signal attributable to misfire can be eliminated.
In other words, since selection of a frequency that is not a multiple (in integer) of the prescribed frequency ensures that the effect of misfire does not appear simultaneously in both the output of the first filter and the output of the second filter, fluctuation of the wheel speed sensor output signal attributable to misfire can be eliminated and solely the wheel speed sensor output signal attributable to rough road running can be extracted. As a consequence, rough road running or other such specific running (operating) condition can be accurately detected to reliably avoid false discrimination of misfire while ensuring that misfire detection is not erroneously disabled when misfire does occur.
In a third aspect of the present invention, the first filter is a band-pass filter that passes a first prescribed frequency component in the output signal of the wheel speed sensor, and the second filter is a band-pass filter that passes a second frequency component that is higher than the first prescribed frequency and that is a multiple of the first prescribed frequency (that is an nth harmonic thereof).
In the third aspect, the second filter is used to select a frequency component whose frequency is higher than the prescribed frequency and, differently from in the second aspect, is a multiple of the prescribed frequency. Therefore, when misfire happens to occur at a frequency equal to the eigenfrequency of the vehicle drive system, a harmonic component that is a multiple thereof is selected by the second filter, and whether the fluctuation of the wheel speed sensor output signal is attributable to misfire or attributable to rough road running or other such specific running (operating) condition can be discriminated with good accuracy utilizing the output of the first and second fluctuation value calculation means obtained using the first and second filters by, for example, dividing the output of the first filter by the output of the second filter. As a consequence, false misfire detection can be reliably avoided without disabling misfire detection owing to erroneous detection of misfire as rough road running or the like.
In the foregoing, the term xe2x80x9cmisfirexe2x80x9d is used to encompass all cases in which combustion does not occur, irrespective of whether because of failure of the fuel supply system or failure of the ignition system, and also irrespective of whether or not spark discharge occurs.